1. Field of the Invention
The present invention relates to a method and apparatus for receiving and processing burst data in an optical line terminal (OLT) of an Ethernet passive optical access network, and more particularly, to a method and apparatus for receiving and processing burst data for improving a burst mode receiving performance in an optical line terminal (OLT).
2. Description of the Related Art
As the number of Internet users using wireless communication has abruptly increased, high-speed Internet technologies have been evolved to provide faster Internet services to users. A wireless local area network (LAN) is one of representative technologies for providing high speed Internet services to users. The wireless LAN has shortcomings of a large gap between a wide area network and an end user and a bottleneck problem of a bandwidth at the end users. In order to overcome the bottleneck program of the bandwidth, a passive optical network (PON) was introduced recently. Such a PON technology may be classified into an ATM PON (APON) and an Ethernet PON (EPON). The EPON technology is a control chip technology that allows users to access a Fiber to the Home (FTTH) network, which is capable of providing various communication services including Internet, Internet TV, digital TV and telephone with affordable cost, through one optical fiber.
FIG. 1 is a diagram illustrating a structure of Ethernet passive optical network (EPON) according the related art. The EPON is a system that transfers a signal to an end user through an optical cable network. The EPON is classified into FTTC, FTTB and FTTH according to a location of termination.
Referring-to FIG. 1, the EPON includes an optical line terminal (OLT) 10 cooperated with a communication service provider side such as an IP network, a broadcasting network, or a TDM network, and an optical network unit (PNU) 20 connected to a user terminal 30 placed at a user end of an optical access network, such as STB or PC. The EPON uses an optical fiber and generally has a point-to-multipoint tree structure. An optical star coupler/splitter 40 may split an optical signal transferred to an Ethernet passive optical network system according to whether the light thereof is upstream or downstream. The split signals are transferred through a plurality of optical fibers, or the split signals are coupled and transferred through one optical fiber. As an example, a method of transmitting data in EPON will be described with reference to FIG. 2.
The EPON transmits data using an Ethernet frame as a transmission unit. In an upstream path, each of ONUs 20 is allocated to a time slot dynamically or fixedly, and transmits data to a common OLT 10. An optical star coupler 40 multiplexes the data outputted from each of the ONUs 20 and transmits them to the OLT 10. ON the contrary, in the downstream path, when the OLT 10 transmits a downstream, the optical star coupler 40 inverse multiplexes the downstream and transmits them to each of the ONUs 20.
The OLT 10 recovers burst data transmitted in a burst mode from an ONU 20 through an upstream. Since the EPON is a technology for high speed Internet service, the Internet speed would be significantly influenced if a time of restoring burst data delays. Such a delay also wastes the bandwidth of the upstream channel. The delay is mainly caused by a time required for restoring transmitting data from an ONU at an OLT, and a laser on/off time of an ONU. A sum of such times except the time of transmitting data is a guard time.
Therefore, it is required to reduce a clock and a data restoring time when the OLT 10 receives burst data. However, a conventional EPON only considers the allocation of synch times.
Generally, a receiver of an OLT in an EPON system uses an Ethernet SerDes chip. According to the type of the Ethernet SerDes chip and a method of using the same, a CDR time of about 200 nano seconds to about several nano seconds is required. The CDR time becomes a synch time by being added to an OLT optical receiving level recovery time. If the CDR time is significantly lengthened, the synch time becomes lengthened, accordingly. Also, if the synch time becomes lengthened, a upstream band would be wasted as much as the lengthened synch time, and if the number of branches increases, the overhead becomes also bigger as much as the increased number of the branches.